In recent years, an optical system for forming an image of an object in a space by using a reflective imaging element has been proposed (for example, Patent Documents 1 and 2). The optical system includes a reflective imaging element and an object, and an image to be displayed in a space is an image of the object, which forms at a position of planar symmetry with respect to the reflective imaging element as a plane of symmetry. This optical system utilizes specular reflection of the reflective imaging element, and, according to its principles, the ratio in size between the image of the object and the image appearing in the space is 1:1.
Disclosed as the reflective imaging element are: those featuring holes penetrating a plate-like substrate in the thickness direction, with an optical element (also referred to as a unit imaging element) composed of two orthogonal specular elements provided on the inner walls of each hole (e.g., see FIG. 4 of Patent Document 1); and those featuring a plurality of transparent cylindrical bodies protruding in the thickness direction of a substrate, with an optical element composed of two orthogonal specular elements provided on the inner wall surfaces of each cylindrical body (e.g., see FIG. 7 of Patent Document 1).
In the reflective imaging elements disclosed in Patent Documents 1 and 2, tens to hundreds of thousands of square holes, each of whose sides measures about 50 μm to 200 μm, are formed in a substrate having a thickness of 50 μm to 200 μm, the inner surfaces of each hole being mirror coated by an electroforming technique, a nanoprinting technique, or a sputtering technique.
For reference sake, the entire disclosure of Patent Documents 1 and 2 is incorporated herein by reference.